1. Field of the Invention
This invention relates to a recording medium to be suitably used with water-based ink for recording and a method of manufacturing the same. More particularly, the present invention relates to a recording medium particularly adapted to ink-jet recording with the effects of a high optical density of images, sharp tones, a gradation of a large number of stages, freedom from changes of tint and any noticeable beading phenomena and an excellent ink-absorption capacity when used for printing at high speed with any of various different types of ink including ink showing different densities, an ink set of three or more than three different densities, ink to be mostly used for solid images, ink of a mixture of pigments/dyes or a combination of pigment ink and dye ink and also to a method of manufacturing such a recording medium.
2. Related Background Art
The ink-jet recording system is a system of causing micro-droplets of ink to fly and adhere to a recording medium such as a sheet of paper for recording images and/or characters on the basis of a selected principle of operation. It provides a number of advantages including high speed/low noise printing, ease of multi-color printing, versatility of patterns that can be recorded and elimination of development and fixation processes. Hence, it has been popularly used in various recording apparatus, particularly in the field of information-related equipment, and the demand for such systems has been expanding rapidly. Additionally, images formed by the multi-color ink-jet recording system are comparable to those produced by multi-color plate printing and color phototypesetting in terms of image quality, and are less costly than ordinary multi-color printing and printing of other types so that the multi-color ink-jet recording system is broadening the scope of application to include full color image recording.
While a number of improvements have been realized for recording apparatus and recording methods employing a multi-color ink-jet recording system to keep pace with the recent developments of recording technologies particularly in terms of high speed recording, high definition recording and full-color recording, sophisticated technological requirements have also been placed for the recording medium.
In an attempt to meet such technological requirements, various forms of recording mediums have been proposed to date.
For instance, Japanese Patent Application Laid-Open No. 52-53012 discloses ink-jet recording paper prepared by causing a surface processing paint to permeate into low sized base paper. Japanese Patent Application Laid-Open No. 53-49113 discloses ink-jet recording paper prepared by impregnating sheets of paper that have been coated with particles of urine-formalin resin with a water-soluble polymeric compound. Japanese Patent Application Laid-Open No. 56-5830 describes ink-jet recording paper comprising an ink absorbing layer formed on the surface of a base material layer by an application process. Japanese Patent Application Laid-Open No. 55-51583 describes the use of amorphous silica as pigment in the coating layer of ink-jet recording paper. Japanese Patent Application Laid-Open No. 55-144172 discloses an image receiving sheet of paper having a layer formed by applying a pigment adapted to absorb the coloring agent of water-based ink. Japanese Patent Application Laid-Open No. 55-146786 discloses the use of a layer of a water-soluble polymeric compound formed by an application process.
Improvements of ink absorption and surface gloss of a paper recording medium also have been proposed in, for example, U.S. Pat. Nos. 4,879,166 and 5,104,730, and Japanese Patent Applications Laid-Open Nos. 2-276670, 3-215082 and 3-281383. Japanese Patent Applications Laid-Open Nos. 7-089221, 7-172038, 7-232473, 7-232474, 7-232475, 8-132731, 8-174993, 9-066664, 9-076628, 9-086035 and 9-099627, of the inventors of the present patent application, propose sheets of recording paper having an ink-receiving layer formed by using hydrated alumina such as pseudo-boehmite.
U.S. Pat. No. 4,879,166, European Patent No. 298,424 and Japanese Patent Applications Laid-Open Nos. 1-97678, 6-48016 and 6-55829 propose a recording medium formed by using both hydrated alumina having a specific absorption ability and silica.
Additionally, the patent documents as listed below propose a two-layered recording medium devised for improving the image quality, the gloss and the surface resistance against scars of recording medium.
(1) U.S. Pat. No. 5,104,730, European Patent No. 407,720, Japanese Patent Applications Laid-Open Nos. 2-276671, 3-281383, 4-115984 and 4-115985 propose a multilayer recording medium having a layer of porous micro-particles of silica formed on a porous alumina layer.
(2) Japanese Patent Application Laid-Open No. 6-18131 proposes a recording medium comprising a first ink-receiving layer formed on a base material layer and a second ink-receiving layer of inorganic micro-particles formed on the first layer and practically not containing any organic polymeric adhesive agent.
(3) U.S. Pat. No. 5,463,178, European Patent No. 634,287 and Japanese Patent Application Laid-Open No. 7-76162 propose a recording medium comprising a porous hydrated alumina layer and a silica gel layer formed thereon.
(4) Japanese Patent Application Laid-Open No. 10-166715 proposes a recording medium comprising a base material layer, an ink-receiving layer of hydrated alumina such as pseudo-boehmite and a silica layer containing non-spherical silica particles.
(5) Japanese Patent Applications Laid-Open Nos. 7-089220, 7-101142 and 7-117335 propose a recording medium comprising upper and lower ink-receiving layers, of which the upper layer is a glossy layer containing colloidal silica as principal ingredient.
(6) Japanese Patent Applications Laid-Open Nos. 9-150571, 9-175000, 9-183267, 9-286165 and 10-71764 propose a recording medium comprising a pair of ink-receiving layers, where the pore distribution and the average particle diameter of the silica particles of the upper layer are limited to respective specific ranges or silica is used in combination with alumina sol or silica alumina for the upper layer.
The above listed patent documents propose improvements of the properties of recording mediums including ink absorptivity, resolution, image density, coloration, color reproducibility, transparence and gloss. Despite the above described improvements and other improvements, a recording medium of the type under consideration faces needs that have arisen, due to the recent technological development in the field of recording apparatus, for high speed printing with a degree of image quality comparable to silver salt photographs. For example, while the recording medium realized by using hydrated alumina or a combination of hydrated alumina and silica as disclosed in U.S. Pat. No. 4,879,166 is excellent in terms of image quality and gloss, it is accompanied by the problem that the surface is apt to be damaged so that the printed surface can easily become scarred depending on the delivery system of the printer. Additionally, the ink absorptivity of the recording medium can be degraded in a hot and humid environment and sheets of the recording medium can stick to each other when stacked for storage in such an environment.
While a recording medium having two ink-receiving layers is proposed in a number of patent documents in order to improve the ink absorptivity and the surface properties, the proposals are accompanied by respective drawbacks as discussed below and hence are not satisfactory.
(1) A multilayer recording medium according to any of U.S. Pat. No. 5,104,730, European Patent No. 407,720, Japanese Patent Applications Laid-Open Nos. 2-276671, 3-281383, 4-115984 and 4-115985 comprises a layer of porous micro-particles of silica formed on a porous alumina layer. The porous alumina layer is intended to absorb the colorant of the ink used for printing, while the silica layer is designed to absorb the solvent of the ink. With this arrangement, although the ink is absorbed well with an excellent coloring effect mainly due to the separated functional roles of the two layers, it is accompanied by the problem that the silica layer becomes white and opaque due to the porous micro-particles of silica of the silica layer.
(2) A recording medium according to Japanese Patent Application Laid-Open No. 6-18131 comprises two ink-receiving layers, of which the surface layer is a layer of inorganic micro-particles formed on the first layer and practically not containing any organic polymeric adhesive agent. While this arrangement provides the advantage that no swelling nor dissolution occurs along the interface of ink and resin due to the ink that comes into contact nor the resin is deformed as a result of printing, it cannot secure a satisfactory level of film strength so that the film can be peeled off and/or damaged when the printer is moved or otherwise handled.
(3) A multilayer recording medium according to any of U.S. Pat. No. 5,463,178, European Patent No. 634,287 and Japanese Patent Application Laid-Open No. 7-76162 comprises a silica gel surface layer. However, since primary silica particles are arranged regularly in the silica gel layer without forming secondary particles, silica particles are filled densely in the layer to eliminate gaps through which the solvent can move, the absorptivity of the recording medium is not remarkably improved by the provision of a silica gel layer on the pseudo-boehmite, porous layer.
(4) A multilayer recording medium according to Japanese Patent Application Laid-Open No. 10-166715 comprises a surface silica layer containing non-spherical silica particles. While this arrangement improve the permeation of ink because particles are filled coarsely there from a microscopic point of view, it is accompanied by the problem of a reduced transparency and a frequent occurrence of cracks due to the use of spherical silica particles.
(5) A multilayer recording medium according to any of Japanese Patent Applications Laid-Open Nos. 7-089220, 7-101142 and 7-117335 comprises upper and lower ink-receiving layers, of which the upper layer is a glossy layer containing colloidal silica as principal ingredient. While this arrangement ensures an enhanced level of surface gloss for the upper ink-receiving layer, it requires the use of a cast molding process to reduce the absorptivity to say nothing of improving the latter. While the proposed recording medium is prepared on the basis of various ingenious arrangements including that of regulating the glass transition temperature of polymeric latex that is also used in the recording medium, that of utilizing colloidal silica composite emulsion and that of reducing the average particle diameter of colloidal silica to less than 300 nm, it cannot prevent the reduction of ink absorptivity because of the use of a cast, although it may be able to alleviate the reduction of porosity to some extent by selecting appropriate operating conditions for the cast.
(6) A multilayer recording medium according to any of Japanese Patent Applications Laid-Open Nos. 9-150571, 9-175000, 9-183267, 9-286165 and 10-71764 comprises a pair of ink-receiving layers, where the pore distribution and the average particle diameter of the silica particles of the upper layer are limited to respective specific ranges, in order to improve both the ink absorptivity and the transparency. However, due of the fact that a wide range is selected for the average particle diameter of the silica particles of the upper layer, it will be difficult to realize a satisfactory level of transparency if the silica particles have large particle diameters or form secondary particles. Additionally, the ink absorptivity of the ink layer may not be sufficient because of difficulties in forming a satisfactorily porous layer if the resin used as adhesive is soluble in water. In short, the recording medium proposed by any of these patent documents cannot provide a level of porosity that ensures both a satisfactory level of transparency and that of absorptivity.
In view of the above identified problems and other problems of the prior art, it is therefore the object of the present invention to provide a recording medium that is adapted to ink-jet recording with the effects of a high optical density of images, sharp tones, a gradation with a large number of stages, freedom from changes of tint and any noticeable beading phenomena and an excellent ink-absorption capacity as well as a high surface resistance against scars and an enhanced level of transparency when used for printing at high speed with any of various different types of ink including an ink set of three or more than three different densities, ink to be mostly used for solid images, ink of a mixture of pigments/dyes or a combination of pigment ink and dye ink, and also to an image forming method using such a recording medium.
In an aspect of the invention, the above object is achieved by providing a recording medium comprising a base material layer, a porous lower layer containing hydrated alumina showing a boehmite structure and a porous upper layer containing silica, wherein said porous upper layer mainly comprises agglomerates of spherical silica particles with a particle diameter between 1 and 100 nm and a binder and voids and said voids are mainly found between said agglomerates and not within the said agglomerates.
According to the present invention, there is also provided a method of manufacturing a recording medium comprising steps of sequentially laying a porous lower layer containing hydrated alumina showing a boehmite structure and a porous upper layer containing silica on a base material layer, wherein said porous upper layer is formed by applying and drying a dispersive solution prepared by adding alcohol by 30 to 90% to an aqueous dispersive solution containing spherical colloidal silica with an average particle diameter between 1 and 100 nm and at least a type of resin emulsion.
A recording medium according to the invention shows improved surface properties and ink absorptivity. A method of manufacturing a recording medium according to the invention can provide an improved recording medium to be preferably used for ink jet recording. The present invention has been realized as a result of research efforts paid by the inventors of the present invention on the basis of their findings.